StandaloneMm: enable serial debug output for diagnostics

Switch DebugLib from Null to BaseDebugLibSerialPort to get edk2
debug prints from StandaloneMm on the serial console. This helps
diagnose initialization failures in the secure partition.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lizthegrey

Platform/SolidRun/StandAloneMm/StandaloneMm.dsc

@@ -37,8 +37,8 @@
   ArmLib|ArmPkg/Library/ArmLib/ArmBaseLib.inf
   BaseLib|MdePkg/Library/BaseLib/BaseLib.inf
   BaseMemoryLib|MdePkg/Library/BaseMemoryLib/BaseMemoryLib.inf
-  # DebugLib|MdePkg/Library/BaseDebugLibSerialPort/BaseDebugLibSerialPort.inf
-  DebugLib|MdePkg/Library/BaseDebugLibNull/BaseDebugLibNull.inf
+  DebugLib|MdePkg/Library/BaseDebugLibSerialPort/BaseDebugLibSerialPort.inf
+  # DebugLib|MdePkg/Library/BaseDebugLibNull/BaseDebugLibNull.inf
   DebugPrintErrorLevelLib|MdePkg/Library/BaseDebugPrintErrorLevelLib/BaseDebugPrintErrorLevelLib.inf
   ExtractGuidedSectionLib|EmbeddedPkg/Library/PrePiExtractGuidedSectionLib/PrePiExtractGuidedSectionLib.inf
   FvLib|MdePkg/Library/FvLib/FvLib.inf